(1) Field of the Invention
This invention relates to a process for recovering Cr(VI) ions from a chlorate cell liquor containing an alkali metal chlorate, an alkali metal chloride and Cr(VI) ions, and for re-using the recovered Cr(VI) ions in an electrolysis step for manufacturing an alkali metal chlorate, said chlorate cell liquor resulting from an electrolytic process for manufacturing an alkali metal chlorate in which an alkali metal chloride is electrolytically oxidized to an alkali metal chlorate.
(2) Description of Prior Art
A chlorate cell liquor contains mainly sodium chlorate and sodium chloride. However, it is conventional to add Cr(VI) ions (By the term "Cr(VI) ions" we mean chromate and/or dichromate.) to the electrolyte for the purpose of suppressing the cathodic reduction of the intermediately formed hypochlorite ions. Because of the toxicity of Cr(VI) ions, discharge of any Cr(VI) ions to the environment should be avoided, since even only small amounts of Cr(VI) ions cause seriously environmental pollution. Moreover, the solid chlorate products crystallized from a chlorate cell liquor containing Cr(VI) ions are colored yellow with the attached Cr(VI) ions, which reduce the commercial value of such products. At present, the attached Cr(VI) ions are removed by water-washing or recrystallization, and a refined aqueous chlorate solution can be produced only by dissolving the recrystallized solid chlorate in fresh water. However, this requires a complicated process and equipment.
Chlorine dioxide is a commercially important material in such fields as pulp bleaching and fat decoloring, and also has recently been used for the purpose of environmental protection and pollution abatement as in the removal of phenols from industrial sewage and in the denitration of industrial waste gases. A process for manufacturing chlorine dioxide by reacting an alkali metal chlorate with hydrochloric acid in a single generator-crystallizer is described in Japanese Patent Publication No. 43401/1980 and Japanese Patent Public Disclosure (Kokai) No. 74296/1979. This process comprises a step of feeding an alkali metal chlorate and hydrochloric acid to a single generator-crystallizer, a step of evaporating water from an aqueous reaction medium in the single generator-crystallizer, therein depositing the corresponding alkali metal chloride, a step of withdrawing a gaseous mixture consisting of chlorine dioxide, chlorine and water vapor and a step of recovering said alkali metal chloride. In this process, a palladium complex is added as a catalyst for manufacturing the valuable chlorine dioxide efficiently. However, if a chlorate cell liquor containing Cr(VI) ions is fed and caused to react with hydrochloric acid in such a single generator-crystallizer, Cr(VI) ions are accumulated and concentrated therein, causing such undesirable effects as the elevation of the reaction temperature due to a boiling point rise and even the poisoning of the added catalyst due to the excess of the accumulated Cr(VI) ions.
It is industrially difficult to separate Cr(VI) ions by water-washing or the recrystallization in that foregoing process. Moreover, Cr(VI) ions are so valuable that it is desirable to recover them and to re-use them in the electrolytic cell.
Processes for recovering Cr(VI) ions by reducing them to Cr(III) ions have been proposed, for example, in Japanese Patent Publication No. 7692/1969, in which Cr(VI) ions are reduced to Cr(III) ions with ferrous salts and removed by forming a precipitate of hydrous chromic oxide from an aqueous chlorate cell liquor containing Cr(VI) ions. With this process, however, it is necessary to reoxidize Cr(III) ions back to Cr(VI) ions. A process for recovering Cr(VI) ions by anion exchange resins was proposed in U.S. Pat. No. 3,835,001. However, in this process regeneration of the anion exchange resin bed adsorbing Cr(VI) ions by passing an aqueous alkali metal hydroxide solution containing an alkali metal chloride through the bed is not efficient, so it is desirable to improve the efficiency of the regeneration.